1. Field of the Invention
The invention relates to integrated circuits for use at microwave frequencies, and more particularly to a switchable bidirectional phase shift network suitable for use as a phase bit in a multi bit phase shifter.
2. Prior Art
The present phase shift network is one of a class of phase shift networks in which the same phase response is required in the forward or reverse direction through the network, and in which the network takes either of two states, as a result of switching action, between which a fixed phase difference persists over a predetermined band of frequencies. A full choice of differential phase shifts at a given level of accuracy is ordinarily desired, and is achieved by the use of a plurality of phase shift networks in a multi bit phase shifter. The individual phase shift networks are usually called "phase bits", and have differential insertion phase values scaled in powers of two to achieve 360.degree. of phase shift coverage. Typically the phase states of the phase shifter are distributed in 32, 64 or 128 (etc.) equal increments around 360.degree. of total differential insertion phase coverage. The largest phase bit will usually represent one-half of 360.degree.. The smallest phase bit will represent 1/32, 1/64, or 1/128 of 360.degree. and will equal the differential phase increment between successive phase shifter states.
The individual "bits" of phase shifters need not be identical. The bits having the smaller differential insertion phase may need to be no more than a serial transmission line in parallel with a transistor switch to meet the requisite performance requirements. On the other hand, the bits exhibiting the larger differential insertion phase may require a relatively complex network. One known network provides two independent filter sections, one of which is operative in one state, and the other of which is operative in the other state. Such a filter is the well-known switchable low pass, high pass filter. This filter includes a pair of single pole double throw switches configured to insert a low pass filter in one state of phase response and to insert a high pass filter in the other state of phase response. This design, with the use of second order networks, can be made to operate over quite substantial bandwidths at microwave frequencies. The switching in such a filter may be provided by four field effect transistors configured to provide single pole double throw action.
The conventional low pass, high pass filter is, of course, too complex for many applications, particularly for smaller differential phase shifts, and in some practical cases at microwave frequencies may not be the best performing circuit.
Economic considerations are also of substantial importance in the design of phase shift networks. The conventional low pass, high pass filter is composed of inductors and capacitors and switches, switching normally being realized by field effect transistors. At microwave frequencies, field effect transistors are not simple switches but exhibit resistive and capacitive parameters. When costs are of concern, it may be preferable to minimize the component count to reduce the size of each "cell" containing a phase bit and thereby reduce the size of a phase shifter chip containing a plurality of phase bits. This consideration is important in MMIC constructions in which the passive components are formed on the surface of a substrate of a costly material such as Gallium Arsenide, also suitable for the formation of the active components. In MMIC applications, the smaller the layout that exhibits the required performance, the lower the circuit cost. A result of this design emphasis is to avoid unnecessary components and to be generally sparing of chip area in the distribution of components over the layout. The present invention applies these considerations to a switchable phase shift network for use in a phase shifter using MMIC fabrication techniques.